Conventionally, a terahertz time-domain spectroscopy (referred to as a THz-TDS method, hereinafter) is known as a typical spectroscopy method using a THz pulse. FIG. 1 shows a typical experimental system configuration according to a THz-TDS method. In this method, first, temporal waveform of a pulsed THz electric field is measured by a pump-probe measurement (or cross-correlation measurement) using a THz pulse and a probe pulse light. The pump-probe measurement is a technique in which as shown in FIG. 2, the timing that the THz pulse and the probe pulse light overlap each other is sequentially shifted by time-delay scanning using a mechanical stage, and in which electric field of THz pulse sampled with the pulse width of the probe pulse light at each overlapped timings is measured on by one so that an ultrafast temporal waveform is reconstructed which cannot be measured in real time.
Further, in the THz-TDS method, Fourier spectra of THz amplitude and phase are obtained for spectroscopy measurement by performing Fourier transform calculation of the temporal waveform of THz electric field with a computer.
FIG. 3 shows the frequency spectrum of the THz amplitude obtained by performing, with a computer, Fourier transform calculation on the measured temporal waveform of THz electric field. Here, when the measured time window is denoted by T, the frequency resolution of the THz amplitude spectrum is expressed by 1/T.
That is, the frequency resolution is determined by the measured time window T of the THz electric field (amount of time-delay scanning), and hence is restricted by the moving stroke length (L) of time delay scanning (a mechanical stage) of FIG. 1. On the other hand, the frequency range is given by the inverse (1/t) of unit increment of time-delay scanning stage (t).
Thus, in the conventional THz-TDS method based on the mechanical time-delay scanning, there is an inherent trade-off between improvement of frequency resolution and a reduction in measuring time.